In a medium pot, heat olive oil over medium-high heat, add onion and cook about 5 minutes until translucent.
Add garlic and season with salt and pepper, continue cooking another 30 minutes.
Add tomatoes with juice and bring to a boil. Reduced to simmer, and cook about 15 minutes.
Transfer to blender and puree until smooth. Be really careful with the hot liquid; remove cap from blender and cover opening with a dish towel. Or use an immersion blender.